Academia appears to have evolved these norms because if papers could be as long as blog posts, how would anyone distinguish the scholars from the hobbyists? Journal papers in the social sciences are longer than blog posts on the same subjects less because academics have more to say than the fact that professors need to justify their positions as credentialed experts rather than amateurs. A lot of smart people can write decent blogs, but no one has the time to track down all the citations you need to be published in the American Political Science Review unless they’re working full-time as a professor. On the surface, more words and more citations signal “I know more about this,” but in reality they just show that one has devoted more time to it, in the hopes that observers can’t tell the difference.
Academia has made a specialty of bad kinds of signalling, but this doesn’t mean that there aren’t good kinds. When you read my articles, I want you to know that I am a smart person who has thought carefully about the issues I write about, and I have some of the basic skills and training necessary to make sense of the world. There are legitimate choices here to be made regarding tone, writing style, and word choice, and other things to consider besides broad accessibility.